Our current geological epoch has supported unusually stable climate, one that allowed human civilizations to live on without major cooling to stymie technological developments. Our current epoch is the Holocene, which began roughly 11,600 years ago. The Holocene is an interglacial, or between-ice-age period, which generally is host to a climate that steadily cools until the next ice age. But the usual interglacial cooling never transpired during the Holocene. This in part is due to an unusual and steady rise in global carbon dioxide (CO2) levels of 20 parts per million (ppm) starting 8,000 years ago, roughly equivalent to the rise in atmospheric CO2 we’ve witnessed between 2009 and today thanks to fossil fuel burning. An explanation for this natural 20 ppm rise has eluded scientists, and in their study geochemist Anja S. Studer of the Max Planck Institute for Chemistry in Mainz, Germany and her colleagues look to the ocean for answers.

Figure 1. Arrowheads estimated to date back to between 5,500 and 6,500 years ago, just after the start of the unusual rise in atmospheric CO2. Source: Jen Jackson of Kent County Council via finds.org.uk.

The ocean, able to absorb vast amounts of carbon dioxide from the atmosphere, can also release the gas back into the air. The Southern Ocean, which encircles Antarctica, is a region where a significant fraction of the total air-sea gas exchange occurs. Positing that the unexplained increase in atmospheric CO2 may have come from the ocean, Studer and her colleagues analyzed sediment samples from the seafloor of the Southern Ocean. These millennia-old samples were extracted from deep within the seabed, and the atoms they contain offer information about the ocean in times long past.

Hoping to uncover information about whether there were any apparent changes happening in the ocean during the time of the CO2 rise, the researchers analyzed the nitrogen atoms held in these old sediment and coral samples. A possible explanation for the CO2 rise is that the water circulating in the ocean was temporarily altered, bringing deep, CO2-rich water up to the surface and leaking the gas back into the atmosphere. This deep water is also rich in nitrogen, and contains a lower ratio of heavy to light nitrogen atoms (known as the isotopes of nitrogen) than the surface ocean. The sediment comprises matter that sank from the surface ocean to the seafloor; if this deep water was mixing with surface waters more than usual, we’d expect to see that the nitrogen in the samples had a lower-than-usual ratio of the heavy to light nitrogen isotope.

Other researchers have attempted to tackle this problem by studying nitrogen in sediment and coral samples, but had only limited success because the ages of the available samples didn’t span the entire Holocene. But Studer and her colleagues provide two new sediment cores containing useful isotopic information, both taken south of the Indian Ocean close to Antarctica (Figure 2). When combining these new records with the ones previously available, the researchers indeed found that the ratio of heavy to light nitrogen decreased before the start of the CO2 rise, supporting their hypothesis that changes in ocean circulation were involved.

Figure 2. Map of location of seafloor samples taken in the Southern Ocean. In their work, Studer and her colleagues present the data taken at sites MD11-3353 and MD12-3396. Source: Studer et al. (2018)

While Studer and her colleagues’ new samples supported their hypothesis, finding other non-nitrogen measurements to either support or refute their explanation proved difficult. This was in part because paleoceanographic data is sparse, but also because atmospheric CO2 levels are influenced by many different natural mechanisms. Despite these limitations, the scientists noted that the change in ocean circulation they detected in their isotope analysis, in conjunction with other changes in land-air CO­2 exchange and carbon chemistry in the ocean, could fully account for the 20 ppm CO2 rise that was observed. And what would have caused the ocean circulation to change? While current hypotheses involve changes in water circulation in the North Atlantic or the patterns of the winds that drive water movement in the Southern Ocean, there isn’t a clear answer.

Though the mechanism responsible for this CO2 rise has not yet been confirmed, Studer and her colleagues’ work offers additional support of the idea that an ocean-related mechanism was involved. The interruption of the expected Holocene cooling brought the extended stretch of livable climate that allowed human societies to innovate the bows, arrows and spears that archaeologists have found from the start of the CO2 rise. But beyond intertwining the ocean’s history with our own, this work helps us to predict how the ocean might affect the climate in the future. Due to our fossil fuel burning, CO2 levels in the last 100 years have been rising at rates never observed in our planet’s history, even while the ocean and land draw down roughly half of the CO2 we emit. While the modest natural addition of 20 ppm has kept our planet livable during the Holocene, changes in ocean circulation might have a very different effect on our climate in the future.

Julia is a PhD student at Scripps Institution of Oceanography in La Jolla, California. Her focus is on biogeochemistry, which, as the name suggests, centers on the combined effects of biological, geological and chemical processes on the earth system. She is advised by Dr. Ralph Keeling and is modeling the global carbon cycle to better understand how much carbon dioxide ends up in the atmosphere. When not at her computer writing code, Julia can usually be found reading and/or thinking about food.